If you want to use powder, you can try Prantal Powder. It is designed for medical use, you should be able to get it at a pharmacy.. It is designed to treat bedsores and severe skin irritations. It was the only thing that helped mum when she was sick.
I used it on Charlotte after she suggested it. Only needed two changes and it was all but gone. Just wash it off thoroughly at each change and make sure that the area is DRY before reapplying.

The other thing is lots of nappy off time (with or without creams etc)

Hope her botobt feels better soon hun!